Spectroscopic detection of trace gases is used in a wide range of applications, including atmospheric monitoring, industrial process and emissions control, and combustion studies. Spectroscopic detection methods offer high specificity, high sensitivity, fast time response, quantitative determination of species concentrations (i.e., absolute calibration), and the possibility of noninvasive or remote sampling. Although broadband light sources can be used in some situations, laser sources are preferable because of their directionality, high spectral brightness (for attaining high sensitivity), and narrow bandwidth (for minimizing interferences).
For applications requiring compact or portable instrumentation, absorption spectroscopy in the mid-infrared (IR) (3-10 .mu.m) and near-IR (0.8-3 .mu.m) spectral regions has been successfully implemented. Ultraviolet (UV) absorption spectroscopy, however, offers several potential advantages, including larger absorption cross sections, lack of significant interference by water vapor, reduced susceptibility to interference fringes, and solar-blind detection using high-sensitivity, low-noise, low-cost detectors that operate at room temperature (typically silicon photodiodes or photomultiplier tubes).
For a number of trace gas species, ultraviolet fluorescence spectroscopy (i.e., laser-induced fluorescence, LIF) can be used instead of UV absorption spectroscopy to extend measurement sensitivity by a factor of 1000 or more.
Unfortunately, wide application of absorption, LIF, and other spectroscopic detection schemes in the ultraviolet (e.g., laser-enhanced ionization (LEI), resonantly-enhanced multi-photon ionization (REMPI), and resonance Raman spectroscopy), for in situ and remote measurements has been prevented by the size, weight, power consumption, complexity, and cost of existing sources of tunable, narrow-bandwidth, UV radiation. At present most applications requiring tunable, narrow-bandwidth, UV radiation rely on nonlinear frequency conversion of dye lasers and of optical parametric oscillators/amplifiers (OPOs, OPAs).
In a typical frequency-converted dye laser, the second harmonic of a q-switched Nd:YAG laser is used to pump an organic dye solution that exhibits gain over a small portion (usually 10-50 .mu.m) of the 550-1100 nm region. The output of the dye laser is then shifted in wavelength using one or more nonlinear processes. The most commonly used nonlinear conversion schemes are second harmonic generation (SHG), sum frequency generation (SFG), and difference frequency generation (DFG). .beta.-barium-borate (BBO), lithium triborate (LBO), and potassium niobate (KNbO.sub.3) are examples of nonlinear crystals commonly used for wavelength conversion. Alternatively, a solid state gain medium such as Ti:sapphire (usually pumped with an argon ion laser) can be substituted for the dye laser, offering much better wavelength coverage (680-1000 nm) than is attainable with any one laser dye. Even greater wavelength coverage is provided by optical parametric oscillators and amplifiers (OPOs and OPAs), which like dye lasers are typically pumped by a frequency-converted q-switched Nd:YAG laser.
What all of the above lasers systems have in common are logistical problems in applications where size, weight, power consumption, cost, and reliability are important considerations. To overcome the physical limitations of existing UV laser systems, a number of groups have developed UV sources based on nonlinear frequency conversion of diode lasers. These sources have found very few applications, however, because they are characterized by low output power and restricted wavelength coverage. Both of these deficiencies stem from the fact that the peak powers, which are available from diode lasers, are limited to only a few Watts.